The present invention relates to a method and system for dispersion compensation of optical signals. In particular, the present invention relates to a method for higher-order dispersion compensation using at least two chirped Bragg gratings to selectively tune the reflection points of two polarization resolved signals, creating a variable higher-order dependent delay.
Present day telecommunication systems require that optical signals be conveyed over very long distances. In an optical communications signal, data is sent in a series of optical pulses. Signal pulses are composed of a distribution of optical wavelengths and polarizations, each of which travels at its own characteristic velocity. This variation in velocity leads to pulse spreading and thus signal degradation. Degradation due to the wavelength dependence of the velocity is known as chromatic dispersion, while degradation due to the polarization dependence is known as polarization mode dispersion (PMD).
Mathematically, the velocity of light xcexd in a waveguide is given by                     v        =                  c          n                                    (        1        )            
where c is the velocity of light in free space and n is the effective index of refraction in the waveguide.
Normally, the effective index, n, of the optical medium is dependent upon the wavelength of the light component. Thus, components of light having different wavelengths will travel at different speeds.
In addition to being dependent upon wavelength, the effective index in a waveguide also may be dependent upon the polarization of the optical signal. Even in xe2x80x9csingle-modexe2x80x9d fiber, two orthogonal polarizations are supported and, in the presence of birefringence, the polarizations travel at different speeds. Birefringence in the fiber may arise from a variety of sources including both manufacturing variations and time-dependent environmental factors. The speed difference results in a polarization-dependent travel time or xe2x80x9cdifferential group delayxe2x80x9d (DGD) between the two different polarization modes within the birefringent fiber. In real optical fiber systems, the magnitude of birefringence and the orientation of the birefringent axes vary from place to place along the fiber. This results in a more complex effect on the optical signal, which is characterized by the concept of xe2x80x9cprincipal states of polarizationxe2x80x9d or PSPs. PSPs are defined as the two polarization states that experience the maximum relative DGD, and they uniquely characterize the instantaneous state of the system.
Polarization mode dispersion (PMD) is measured as the distortion arising from the statistical sum of the different group velocities of the two components of polarization as the signal propagates through the different sections of the optical communications system. PMD includes first order PMD and higher-order PMD and is non-deterministic. First order PMD is the differential polarization group delay at a given wavelength. The instantaneous value for a long fiber may vary over both long time intervals, due to slow variations such as temperature drift, and short time intervals, due to fast variations such as mechanical vibration induced polarization fluctuations. The coefficient describing the mean value of first order PMD may vary from more than 2 ps km1/2 for relatively poor PMD performance fiber to less than 0.1 ps/km1/2 for relatively good PMD performance fiber.
Second order PMD arises mainly from two sources: i.) a first order PMD that varies with wavelength; ii.) a change of the system PSP (principal state of polarization) orientation with wavelength, which results in a variation of PMD with wavelength. Second order PMD results in a wavelength dependent group delay, which is equivalent in effect to variable chromatic dispersion, and can have either a negative or positive sign. The speed of fluctuation is on the same order as the speed of fluctuation of first order PMD.
There are two types of chromatic dispersion: deterministic and variable. Deterministic dispersion is the set chromatic dispersion per unit length of waveguide having a fixed index of refraction. Deterministic dispersion is relatively fixed (e.g., xcx9c17 ps/nm*km for standard single mode fiber) for a given set of environmental conditions. For example, 17 ps/nm*km means that a ten kilometer (10 km) system, carrying data with a bandwidth of 0.1 nanometers (nm), will experience approximately 17 picoseconds (ps) of chromatic dispersion.
Variable chromatic dispersion is caused by changes in fiber link length, due to adding or dropping channels for example, and by tensile stresses and/or fluctuations in temperature. Reasonable values to be expected for the amount that the chromatic dispersion will change are in the range xe2x88x92500 ps/nm to +500 ps/nm.
In addition to the effects of PMD and chromatic dispersion alone, there is a higher-order dispersion cross term that arises from the simultaneous presence of both chromatic dispersion and PMD. This cross term between chromatic dispersion and second order PMD has a mean value of zero, but may have a non-zero root-mean-square (RMS) contribution. Similarly to second order PMD terms, the RMS value may have a positive or negative contribution. The magnitude of the RMS contribution may vary from less than 1% of the chromatic dispersion to the same order as the chromatic dispersion, depending on the PMD coefficient of the fiber.
Dispersion imposes serious limitations on transmission bandwidth, especially across long distances, such as in transoceanic routes. Dispersion issues become much more important at higher bit rates, where the separation between the optical pulses is less and where shorter pulses result in a wider signal spectral bandwidth, exacerbating chromatic and higher-order PMD effects. At bit rates greater than or equal to 40 Gb/s, even for xe2x80x9cgoodxe2x80x9d fiber (xe2x89xa60.1 ps/km1/2 PMD coefficient), long length links are deemed to require higher-order dynamic compensation. Dispersion may become an inhibiting factor either limiting overall system length or increasing system costs due to the need for additional optical-to-electrical-to-optical signal conversion sites to permit electrical signal regeneration.
Higher-order dispersion has not been adequately recognized, measured and addressed in past dispersion compensation devices. An understanding of the sources and factors in higher-order dispersion is important in providing a higher-order dispersion compensation solution.
The second order coefficient of PMD may be calculated based on the theory described in xe2x80x9cSecond-Order Polarization Mode Dispersion: Impact on Analog and Digital Transmissions,xe2x80x9d IEEE J. of Lightwave Tech., JLT-16, No. 5, pp. 757-771, May 1998, which is hereby incorporated by reference.
Second order PMD coefficient=(First order PMD coefficient)2/1.73xe2x80x83xe2x80x83(2) 
Equation 2 only accounts for the root-mean-square (RMS) of the resulting chromatic dispersion. The cross term was calculated to be:
Cross term=(17)1/2*(First order PMD coefficient)1/2*1.16xe2x80x83xe2x80x83(3)
Therefore, it may be appreciated that for fiber that has a high PMD coefficient, PMD may cause a problem when only fixed chromatic dispersion compensation is used due to accumulated chromatic dispersion through the second order PMD term and the cross term. This leads to a high value of uncompensated dispersion as fiber PMD coefficients become larger or as the bit rate gets higher.
From this analysis, it may be calculated that even using the best of fiber produced today (assuming xcx9c0.025 ps/km1/2), propagation distances are likely limited to ≲3000 km (dispersion  less than 0.3*100 ps) for 10 Gb/s transmission and ≲200 km (dispersion  less than 0.3*25 ps) for 40 Gb/s without performing dynamic chromatic dispersion compensation to eliminate the effects of the 2nd order PMD and cross terms.
A number of literature articles attempt to address the issue of higher-order dispersion compensation. One approach is to use a multi-section PMD compensator. Such an approach is likely to be expensive and also will be limited in the amount of variable chromatic dispersion compensation available. Another approach is to selectively add specific chirps to various portions of the pulse and to send the pulse through a high dispersion element with the correct sign to compress the pulse. Such an approach may account for all types of dispersion. However, such an approach is likely to be expensive due to the need for clock recovery and phase modulation and also only may be useable at the receiver terminal. Furthermore, it only may work if the residual dispersion is low.
The need remains for a dispersion compensation system that dynamically adjusts not only for PMD, but also for chromatic dispersion and higher-order dispersion. Increased telecommunications system requirements, such as the need to compensate for fluctuations in temperature and the possibility of variable path lengths due to the optical add/drop systems envisioned in the near future, call for a compensation system that is dynamic and cost-efficient.
The present invention relates to a higher-order dispersion compensator for tuning a signal having a first polarization mode dispersion component, a second order polarization mode dispersion component, and a variable chromatic dispersion component.
The compensator includes a first tuning element that adjusts the first order polarization mode dispersion component of the polarization controlled signal and a second tuning element that adjusts the second order polarization mode dispersion component and the variable chromatic dispersion component of the polarization controlled signal. The compensator may further include a polarization controller that converts incoming light of an arbitrary polarization to a polarization controlled signal having a desired state of polarization.
In one embodiment, the first tuning element may comprise a differential higher-order delay line including a polarization beam splitter/combiner coupled to receive the polarization controlled signal, where the polarization beam splitter splits the polarization controlled signal into a first polarization component and a second orthogonal polarization component. A first waveguide having a first grating tuned to reflect the first polarization component and a first reference reflection point is optically coupled to receive the first polarization signal. A second waveguide having a second grating tuned to reflect the second polarization component and having a second reference reflection point is optically coupled to receive the second polarization signal. The first waveguide has a first chirped grating tuned to reflect the first polarization signal at a first reference reflection point. The second waveguide is optically coupled and aligned to the second output port of the splitter. The second waveguide has a second chirped grating tuned to reflect the second polarization signal and has a second reference reflection point. Depending on the embodiment of the present invention, the chirp of the gratings may be linear, non-linear, or may have a more complex spatial dependence. For certain applications, the length of the gratings may be equal or greater than one meter.
In this first exemplary embodiment, both gratings are Bragg gratings linearly chirped to perform first order PMD compensation and fixed chromatic dispersion compensation. A first tuning mechanism tunes one of the gratings, such as by mechanically stressing the gratings.
In other embodiments, both gratings may be non-linearly chirped to perform first and second order PMD compensation as well as both fixed and variable chromatic dispersion compensation. Other, more complicated, chirp patterns may be chosen to perform more specific or higher-order (third, fourth, etc. order) compensation.
The first grating and the second grating may both have substantially same reflection profiles and substantially same chirp rates; and the first and second reference reflection points may be at substantially a same optical path length. Alternatively, prior to adjustment by the tuning mechanism, one of the reflection points of the gratings may be at a shorter optical path length from the split point than the second reflection point.
The second tuning element may include a third waveguide having a third non-linearly chirped Bragg grating and a second tuning mechanism that tunes the third grating. The range of chirp values in the third non-linearly chirped Bragg grating may determine the relative range of variable chromatic dispersion compensation.
The system may further include a static chromatic dispersion component comprising an average chirp rate of the first and second gratings that corresponds to the amount of fixed chromatic dispersion to be compensated.
Circulators may be used to route the optical signals. One embodiment includes a four-port circulator, the circulator having an input port optically coupled to receive the polarization control signal, a first recirculation port optically coupled to transmit the controller output signal to the differential polarization delay line and to receive the delay line output, a second recirculation port optically coupled to transmit the delay line output signal to the second tuning element and to a second tuning element output signal, and an output port optically coupled to transmit a final output signal.
In another exemplary embodiment, the first tuning element and the second tuning element comprise a polarization beam splitter coupled to receive the polarization controlled signal, where the polarization beam splitter splits the polarization controlled signal into a first polarization component and a second orthogonal polarization component. A first waveguide is optically coupled to receive the first polarization signal, the first waveguide having a first non-linearly chirped grating tuned to reflect the first polarization signal and having a first reference reflection point. A second waveguide is optically coupled to receive the second polarization signal, the second waveguide having a second non-linearly chirped grating tuned to reflect the second polarization signal and having a second reference reflection point. A first tuning mechanism tunes both the first and the second grating simultaneously and a second tuning mechanism tunes the second grating independently of the first grating. The compensator may have a static chromatic dispersion compensation element, wherein the average chirp rate of the first and second gratings correspond to the amount of fixed chromatic dispersion to be compensated.
In this embodiment, the first order polarization mode dispersion compensation element includes the second tuning mechanism and the second grating and the first order polarization mode dispersion compensation is achieved by tuning the second non-linearly chirped grating separately from the first grating. The second order polarization mode dispersion compensation and variable chromatic dispersion compensation elements include the first and second grating and the first tuning mechanism and variable chromatic dispersion compensation and higher-order polarization mode dispersion compensation are achieved by tuning the first and second gratings in unison.
In yet another embodiment, the higher-order dispersion compensator comprises a chromatic dispersion compensator coupled to receive an input signal; a phase modulator optically coupled to the chromatic dispersion compensator, wherein the phase modulator selectively chirps portions of the data pulses; and a tunable dynamic dispersion element coupled to receive the phase modulated signal. The tunable dynamic dispersion element includes a first waveguide having a first non-linearly chirped grating tuned to reflect the polarization controlled signal and having a first reference reflection point; and a first tuning mechanism that tunes the first grating.
The compensator may include a signal analyzer optically coupled to evaluate the signal reflected by the grating and provide control signals to the tuning mechanism accordingly. The signal analyzer further may provide control signals to the phase modulator.
The waveguides are exemplarily optical fibers. In specific embodiments, the waveguides may be optical single-mode polarization-maintaining (PM) fibers, polarizing (PZ) fibers, and/or shaped optical fibers.
The compensator may be an adaptive compensator further including a signal analyzer, which provides control signals to at least one of the tuning mechanisms.
The dispersion compensator may be at least partially integrated into an integrated optical chip, such as a lithium niobate chip. The waveguides may be channel waveguides. Alternative tuning mechanisms may tune the gratings acoustically, thermally, electro-optically, or mechanically.
A method for compensating for higher-order dispersion of an incoming optical communications signal in accordance with the present invention includes the steps of compensating the signal for first order polarization mode dispersion; compensating the signal for second order polarization mode dispersion; and compensating the signal for variable chromatic dispersion. Additionally, the method may include the steps of compensation for fixed chromatic dispersion and controlling the polarization of the incoming signal. The signal may be monitored after the compensating steps and the degree of compensation may be tuned based on the monitoring.
The step of compensating the signal for first order polarization mode dispersion may include the steps of controlling the polarization of the signal; splitting the signal into a first and a second orthogonal polarization components; reflecting the first polarization component in a fixed linearly chirped grating; reflecting the second polarization component in a tuned linearly chirped grating; and recombining the first and the second polarization components.
The step of compensating the signal for second order polarization mode dispersion may comprise the step of reflecting the signal in a tuned non-linearly chirped grating.
In a particular embodiment of the method of the present invention, the method includes the steps of:
adjusting the state of polarization of the incoming optical communications signal to correctly align the principal states of polarization of the communications signal to the principal states of polarization of the compensator system;
splitting the communications signal into a first and a second orthogonal principal states of polarizations at a split point;
directing the first of the polarization states to a first waveguide having a first non-linearly chirped grating having a first reference reflection point;
directing the second of the polarization states to a second waveguide having a second non-linearly chirped grating having a chirp pattern substantially similar to that of the first chirped grating and having a second reference reflection point;
adjustably varying the chromatic dispersion of the first and second reflections by varying the position of the first and second reflection points along the gratings;
adjustably varying the relative optical path lengths between the first and second reflection points and the split point to compensate for polarization dispersion between the first and second orthogonal states of polarization; and
recombining the first and second polarization states into an output signal.
The method may further include the steps of sampling the quality of the output signal. Using the quality readings, the method may include the steps of:
adaptively adjusting the state of polarization of the incoming signal and the optical path length of the second reflection point with respect to the split point to compensate for first-order polarization mode dispersion in response to the quality of the output signal, and/or
adaptively adjusting one or both of the first and second reflection points with respect to the split point in order to compensate for the dispersion in the signals.
Prior to the step of adjustably varying the optical path length from the second reflection point, the optical path length of at least one of the gratings may be tuned such that the second reflection point is at a desired point, for example, at substantially the same optical path length or at a different path length from the split point as the first reflection point. The difference may be selected according to an expected polarization dispersion delay between the first and second orthogonal states of polarization.
In another embodiment of a method for compensating for higher-order dispersion of an optical communications signal in accordance with the present invention, the method comprises the steps of:
splitting the communications signal into a first and a second orthogonal principal polarization states;
directing the first polarization state to a first high-birefringence optical waveguide having a first linearly chirped grating, the first optical waveguide having a first reflection point at a first optical path length;
directing the second polarization states to a second tunable high-birefringence optical waveguide having a second linearly chirped grating, the second optical waveguide having a second reflection point at a second optical path length;
recombining the first and second polarization states into an output signal;
directing the output signal to a third high-birefringence optical waveguide having a non-linearly chirped grating with a reflection point;
adjustably varying the second optical path length of the second linearly chirped grating to compensate for polarization dispersion between the first and second orthogonal states of polarization; and
adjustably varying the optical path in the third grating to compensate for higher-order dispersion in the output signal.
The second chirped grating may have a chirp pattern substantially similar to the first chirped grating, the second grating having a second reflection point that is substantially at the same optical path length from the split point as the first reflection point. Again, the output signal ay be sampled the state of polarization of the incoming signal and the optical path length of the second reflection point may be adjusted in response to the quality of the output signal.